Against a sophisticated attacker, nothing connected to the internet is secure. Not your GMail account, not your Facebook account, not your website, not your home computer (especially if you are using WiFi), not industrial facilities, not governments.
While this may not absolutely always hold, I am increasingly convinced that the right way to treat the internet is to act as if this is so. If there is some information you absolutely want to keep private, keep it in a form that is not linked to the internet. Dig out an old computer for non-networked use or, better yet, use paper. Accept that anything you put online, even in a private email, could end up on display to the entire world.
People can certainly do a lot to protect themselves from what are essentially untargeted attacks. The people who run botnets just need control of random computers, and their attack methods are good enough to breach security on your average system. If security in yours is significantly better than average, you are probably at little risk from such annoyances. Everything changes, however, when the attacker has resources and expertise at their disposal, and they have you for a specific target. Organizations like governments, corporations, and organized crime groups have these resources, and attack techniques are always spreading to less sophisticated operators. As they say at the NSA, “Attacks always get better; they never get worse.”
Similarly, it is safest to assume that there is no mechanism that you can use to secure a non-networked computer from a sophisticated attacker. You can use encryption, but chances are they will be able to pull the passphrase from somewhere or find some workaround. If that passphrase is short, it can be defeated using brute force dictionary attacks. If it is stored anywhere on your computer, phone, or the internet, it can be found.
If you want secure encryption, use something like random.org to generate a random alphanumeric string with as many bits of data as the encryption you are using (there is little point in using 256-bit AES with a weak key like ‘AnteLope2841’. You need a key like:
Once you have a strong key, write it down on paper, keep it locked up, and never use it for anything other than decrypting that one file.